Conventional photomultipliers (PMTs) are large (>1 cm3), stand-alone devices that exhibit high gain for low-light-level detection. The best PMTs can detect a single photon, and some PMTs have response times of 1 nanosecond or less. The costs of conventional PMTs have fallen to about $500 per device. To date, PMTs have not been integrated onto chips, even though at least two designs have been proposed (see U.S. Pat. No. 5,568,013 and U.S. Pat. No. 5,264,693). The size and cost of traditional PMT's precludes their use in such applications as imaging and optical communications.
Ultrasensitive optical detection is desirable for optical sensors, spectral analysis, imaging, optical receivers and fluorescent microscopy. For example, some recent efforts to develop DNA-sequencing chips utilize fluorescent, site-specific molecular tags attached to the DNA backbone. Optical excitation of the DNA molecule with tags gives characteristic fluorescence that helps describe the structure of the DNA molecule. Since the fluorescence comes from single molecular tags, the light level is low and sensitive detectors are needed. Fluorescent tagging is used in a variety of biological assays to determine the presence of certain species. In spectroscopic applications, PMTs are used to detect faint spectral lines emitted from excited molecules or atoms. PMTs are also used in scintillation studies.
Two designs of integrated photomultipliers have been disclosed previously in patents. One design, proposed in U.S. Pat. No. 5,264,693, is depicted in FIG. 1. For this design, a photocathode 130 and dynodes 140 are enclosed in a large wet-etched chamber 120 fabricated on a planar substrate 160.
The device functions in the following manner. An incident photon passes through the transparent chamber cover 110 and strikes the photocathode 130. The interaction of the photon with the photocathode results in the emission of an electron into the vacuum chamber 120. An applied voltage accelerates the electron to collide with the first dynode. This collision results in the emission of several electrons, providing electron amplification. The amplification is repeated from dynode to dynode and the signal is measured as electron current at the final anode 150.
Unfortunately, the chamber 120 must be under high vacuum, and therefore the covering layer 110 may collapse on the dynodes and cathode unless it is made sufficiently thick. Increasing the cover thickness will be timely and costly. The photocathode 130 and dynodes 140 are patterned before the wet-etching step that defines the photomultiplier chamber. This can be a fatal flaw for such a device, since wet etching will ruin most photocathode materials. The high-efficiency photocathodes must be handled in a pure, high-vacuum environment.
A second design was proposed in U.S. Pat. No. 5,568,013, and is depicted in FIG. 2. This proposed design calls for wide (4 microns wide or greater) channels 220, and requires bonding of top 210 and bottom 270 covers as indicated in the diagram. The photocathode 230 can be patterned on the underside of the top cover, and the anode 250 may be patterned on the top side of the bottom cover. The long channels 220 act as a continuous dynode providing electron amplification as the electrons collide with the channel walls while traveling from cathode to anode.
The bonding step, required to fasten top and bottom covers, is time consuming and requires careful alignment of the covers to the channels. Additionally, since the photocathode has been patterned on the top cover, the bonding and alignment must be done in a high-vacuum environment to avoid ruining the cathode material. The bonding step requires high temperatures, which may also degrade device performance. The bonding procedure is also susceptible to vacuum leaks, should a small particle exist between the cover and substrate. Additionally, this design requires that a hole be fabricated through the substrate above the anode. Such deep etching can be costly and time consuming.
A third photomultiplier design has been proposed in the literature (Charles P. Beetz, et al, Nucl. Instr. Meth. Phys. Res. A, vol. 442 (2000) 443), and has been fabricated. In this design, depicted in FIG. 3, multiple parallel holes 320 are etched through a thin membrane of silicon 360. The walls of the long tubular holes are coated with appropriate material via chemical vapor deposition (CVD) to produce multiple parallel continuous dynodes for electron amplification. The resulting structure functions as a microchannel plate detector.
In Beetz et al. the CVD step restricts the type of materials that may be deposited on the tube walls. Also, only straight long holes are permitted for this device due to the fabrication technique. In some applications, curved channels would improve device performance. Again the etching of long narrow holes through the substrate can be time intensive and costly. Most importantly, for completion this device requires the assembly of separate physical elements, i.e. top cover 310 with cathode 330, electron-amplifying plate 360, and bottom cover 370 with anode or read-out array 350. This assembly must be done in a manner that preserves high vacuum inside the holes. The patent does not describe a method for vacuum-sealing the device, and tests of the submicrochannel plate were done in a vacuum environment.